You can attach a mag clutch
to the compressor drive on the engine
, an overrun clutch
on the compressor itself and when you want to pump air, flip the switch and the mag clutch engages, like an A/C compressor has, and when you're done, flip it off and the over-run clutch on the scuba
air compressor spins freely.
Helped install one on my friends boat. Incredible. I have the Max Air 35 in the Honda version and use it quite alot. I pump Nitrox (many different percents up to 40% as necessary for a specific dive) and mixed gases up to 35/30 trimix.
I paid $3500 for a brand new version 4 years ago.
I buy air filter cartridges and change it after every 30 tank fulls of plain air, less if I pump the other stuff.